This invention relates to aircraft warning systems and more particularly to arrangements for determining the conditions to inhibit false warnings of a ground proximity warning system.
An aircraft generally uses a ground proximity warning system (GPWS) to alert its flight crew to conditions that could result in aircraft crashes due to the position of the aircraft with respect to the terrain along its flight path. GPWS is designed to generate warnings if an aircraft enters a flight path to the ground that could lead to a potentially dangerous situation. Such hazardous conditions could result from an excessive descent rate, an excessive terrain closure rate, loss of altitude after takeoff, insufficient terrain clearance when not in a landing configuration or descent below the instrument landing system (ILS) glide-slope. The GPWS uses inputs from systems that provide indications of radio altitude, aircraft position, airspeed/Mach number, landing gear and flap position and decision height (DH) setting. In situations where an aircraft is deemed too close to the ground, the GPWS system issues an audible and/or visual alarm. In response, the flight crew immediately changes the flight path of the aircraft to avoid a potential crash.
The GPWS system usually relies on a radio altimeter using radar to track the position of the aircraft with respect to the ground. The radio altimeter determines the altitude of the aircraft by reflecting radio waves from the ground. There are, however, numerous reported GPWS nuisance alarms caused by false radio altimeter tracking. The rapid upward pitching of the aircraft when the flight crew responds to a nuisance alarm results in avoidable passenger and flight attendant discomfort.
When the aircraft is not in a landing configuration, a warning issued for insufficient terrain clearance may be generated if there is false radio altimeter tracking. Such false tracking can occur, for example, when the aircraft is flying through severe rain and/or hail conditions or when the aircraft is overflying another aircraft. The returns from the aircraft radio altimeter system can reflect the position of the rain and/or hail condition or the underlying aircraft and falsely indicate that the aircraft is too close to the ground so that a GPWS warning is issued.
FIG. 1 illustrates two conditions under which a radio altimeter may return false indications of altitude. In FIG. 1, an aircraft 105 is at a safe height D1 over the terrain 100 but is flying above an area 110 exhibiting heavy clouds, severe rain and/or hail. Radio altimeter signals are returned to the aircraft 105 that indicate a distance D2 from a severe cloud condition 110. The indication of the aircraft radio altimeter of the D2 height of aircraft 105 above the condition 110 may trigger a GPWS warning. Aircraft 115 in FIG. 1 is also at a safe height D1 over the terrain 100 but is passing over another aircraft 120. Its radio altimeter returns signals indicating a height D3 over the terrain 100 due to the presence of the aircraft 120 and the false indication may trigger a warning from the GPWS system in aircraft 115. In these situations, the aircraft crew must respond by changing aircraft flight path to avoid an apparent hazard. Such false indications of an aircraft""s radio altimeter present a serious problem to safe and comfortable aircraft flight.
The invention is directed to inhibiting false ground proximity warnings of an aircraft resulting from misleading aircraft radio altimeter indications. According to the invention, a radio altitude signal for the aircraft and a separately determined signal representing the clearance of the aircraft from the underlying terrain are generated. In response to the radio altimeter altitude signal and the terrain clearance signal, an indication of the reasonableness state of the radio altitude signal is issued.
According to one aspect of the invention, an indication of an unreasonable state of the radio altimeter signal is issued in response to a comparison of the terrain clearance signal and the radio altitude signal.
According to another aspect of the invention, an indication of an unreasonable state of the radio altimeter signal is issued in response to the terrain clearance signal less the radio altitude signal being larger than a preset value.
According to another aspect of the invention, issuance of a radio altitude unreasonableness state indication is inhibited when the separately determined terrain clearance signal is less than a predetermined value.
According to yet another aspect of the invention, the terrain clearance signal is formed by generating a signal representative of the geometric altitude of the aircraft, generating a signal representative of the height of the terrain underlying the aircraft and producing a signal corresponding to the difference between the geometric altitude and the terrain height.
According to yet another aspect of the invention, the terrain height signal is generated from a terrain database that includes signals representative of the maximum heights of sections of the terrain under the flight path of the aircraft.
According to yet another aspect of the invention, the geometric altitude signal corresponds to the altitude of the aircraft relative to sea level and is generated from Global Positioning System signals received by the aircraft.
According to still another aspect of the invention, a signal representative of the validity of the aircraft position and terrain data is formed. If the aircraft position or terrain data is in an invalid state, the indication of reasonableness of the radio altitude signal is inhibited.
In an embodiment of the invention, signals corresponding to the geometric altitude of an aircraft relative to sea level and the location of the aircraft are generated responsive to data from a Global Position System receiver. A terrain clearance signal is formed from the difference between the geometric altitude and data of the height of the underlying terrain in a terrain database. A processor checks whether flight path data, terrain data and vertical and horizontal data are valid. The aircraft""s radio altitude signal is compared to the terrain clearance signal and an indication of unreasonableness of the radio altitude signal is issued when the terrain clearance less the radio altitude is larger than a first predetermined amount. The indication of unreasonableness is inhibited when the aircraft is below a preset altitude or when any one of the terrain database data, latitude or longitude data, or altitude data is determined to be invalid.